Conventionally, various types of heaters have been proposed (see e.g. Patent Document 1). FIG. 5 shows an example of conventional heater. The heater X illustrated in the figure includes a substrate 91, a heating resistor 92 and a protective film 93. The protective film 93 is for protecting the heating resistor 92 and made up of an inner layer 93a and an outer layer 93b. The inner layer 93a is made of crystallized glass and held in contact with the heating resistor 92. The outer layer 93b is made of amorphous glass and covers the inner layer 93a. The inner layer 93a prevents the heating resistor 92 from being unduly in electrical connection with a conductive part outside the heater X. By making the inner layer 93a using crystallized glass, the withstand voltage of the inner layer (and hence the protective film 93) is enhanced. By making the outer layer 93b using amorphous glass, the obverse surface of the protective film 93 is made smooth.
However, in the conventional structure, the edge of the outer layer 93b made of amorphous glass is held in contact with the upper surface of the substrate 91. Generally, amorphous glass easily form bubbles by reacting with e.g. AlN. Thus, when the substrate is made of AlN in the conventional structure, bubbles may be formed at the edge of the outer layer 93b. The formation of bubbles undesirably reduces the withstand voltage of the protective film 93. Further, moisture in the air is easily absorbed in the inner layer 93a through the portion of the outer layer 93b in which bubbles are formed. The absorption of moisture may cause such a problem as local expansion of the inner layer 93a. 
Patent Document 1: JP-A-2002-289328